Abstract

In spite of varying or opinions as to the effectiveness of
molluscicides in the control of schistosorniasis, the
search for more effective and highly selective
molluscicides should continue, so that adverse effects on
non-target organisms caused by commercially available
molluscicides may be reduced and the possibility of
resistance guarded against.
To this end, the differences in susceptibility to Frescon
and 4'-chloronicotinanilide between the schistosome-bearing
snail, Bulinus truncatus and a representative
tropical food fish, Sarotherodon mossambicus, were
examined and discussed in relation to the rate of uptake
of these compounds and. their distribution among different
organs.
The rate at which water is taken up by B. truncatus and
S. mossambicus has also been calculated and related to the
rate of uptake of molluscicidos. This part of the work
was an attempt to elucidate the mechanism of carriage of
the chemicals into the animals being studied.
It has been demonstrated that B. truncatus and
S. mossambicus concentrate Frescon and 4'-chloronicotinanilide
to high levels. For B. truncatus, both Frescon
and 4'-chloronicotinanilide were concentrated in the
psoudobranch, while in S. mossambicus the highest
concentration of Frescon was found in the liver and that
of 4'-chloronicotinanilide in the bile.
The high tolerance of S. mossambicus to 4' -chloronicotin
anilide was found to be due to the rapid metabolism of
the compound in fish to more polar forms which are more
easily disposed of-via the bile.
Comparison of the relative susceptibilities of two field
collections of B. truncatus showed that snails collected
from the Frescon--treated area of the Gezira display a
higher tolerance to Frescon than do snails collected from
the untreated area. Frescon uptake rate was found to be
lower in the less susceptible snails, and this is
tentatively suggested as the basis of the observed
difference in tolerance. It is additionally shown that
B. truncatus infected with Schistosoma haematobium is
more susceptible to Frescon than uninfected snails.